1. Field of the Invention
The invention relates to a memory circuit having resistance memory cells, in particular having conductive bridging random access memory (CBRAM) resistance memory cells. The invention furthermore relates to a method for reading out a memory datum from such a memory circuit.
2. Description of the Related Art
By way of example, memory circuits may have resistance elements as memory cells, the respective resistance value of which resistance elements may be different depending on the information to be stored. In this case, the resistance elements are arranged in a matrix of word lines and bit lines as is generally customary in memory circuits, and the information is determined by applying a voltage to the word line and measuring the resulting current on the bit line. The bit line current determined is compared with a reference value, and the value of the stored datum is determined in accordance with the result of the comparison. Reference resistance elements are usually provided for this purpose, the resistance value of said elements corresponding to a resistance threshold value, whereby a limit between two states to be stored is defined.
Customary resistance elements include CBRAM resistance elements having a solid electrolyte, for which different resistance values may be set by applying a write current in the CBRAM resistance element. Resistance elements of this type are also referred to as PMC resistance elements (programmable metalization cell) in the literature. Such elements, as already described, are preferably read by applying a voltage across the CBRAM resistance element and by determining the resulting current through the CBRAM resistance element with a voltage drop that is kept constant. For comparison with a reference value, provision is made of a reference resistance element having a resistance threshold value. A sense amplifier circuit is preferably provided for the reference resistance element, the sense amplifier circuit being identical to the sense amplifier circuits for the CBRAM resistance elements. The corresponding output quantities of the sense amplifier circuit for the reference resistance element and the CBRAM resistance element are compared with one another. Since one or more reference resistance elements are essentially provided for each bit line present, there is a considerable additional outlay on circuitry in order to generate the comparison quantity dependent on the resistance threshold value of the reference resistance element and to compare it with the electrical quantity dependent on the memory datum stored in the CBRAM memory cell. In addition, various electrical parameters of the respective sense amplifier circuit for the CBRAM memory cells and for the reference resistance elements influence the corresponding electrical quantities, such as, for example, the parameter of the offset of an operational amplifier used in the sense amplifier circuit. The separation between the resistance values for the individual states of the memory datum that are to be stored therefore has to be chosen to have a sufficiently large magnitude.